Lateral high-resistance additive for zinc oxide varistor, zinc oxide varistor produced using the same, and process for producing the varistor

ABSTRACT

The invention aims at providing highly reliable zinc oxide varistors through simple production steps. The varistor is produced by dispersing a powdery raw material comprising 1-40 molar % (in terms of Fe 2  O 3 ) iron, 0-20 molar % (in terms of Bi 2  O 3 ) bismuth, and the balance consisting of SiO 2  in a solution of a water-soluble binder such as polyvinyl alcohol, and applying the formed dispersion to a molded or calcined zinc oxide varistor to form on the lateral face thereof a lateral high-resistance layer (2) containing Zn 2  SiO 4  as the principal ingredient and a solid solution of iron in Zn 7  Sb 2  O 12  as the auxiliary ingredient.

TECHNICAL FIELD

The present invention relates to a lateral high-resistance additive for forming a lateral high-resistance layer of a zinc oxide varistor mainly used in the field of electric power, a zinc oxide varistor using the same, and a process for producing the zinc oxide varistor.

BACKGROUND ART

A conventional process for producing a zinc oxide varistor is disclosed, for example, in Japanese Laid-open Patent No. 61-259502, and its procedure is as follows.

First, ZnO is a principal ingredient, and small amounts of metal oxides such as Bi₂ O₃, Co₂ O₃, MnO, Cr₂ O₃, Sb₂ O₃, NiO, and Al₂ O₃ are added as auxiliary ingredients. Mixing them sufficiently together with water, binder, and dispersant, slurry is prepared, which is dried and granulated by a spray dryer, and the obtained power is formed in a disk of 55 mm in diameter and 30 mm in thickness. After baking at 500° C. in order to remove organic matter, it is calcined at 1020° C., and a calcined material is obtained. A prepared slurry for forming a high-resistance layer is applied on this calcined material by means of a spray gun.

This slurry for forming a high-resistance layer is prepared by reacting Fe₂ O₃, ZnO and Sb₂ O₃ to produce ZnFe₂ O₄ and Zn₇ Sb₂ O₁₂, weighing powder of ZnFe₂ O₄ and Zn₇ Sb₂ O₁₂ so that the ratio of Fe and Sb may be 2:1, adding purified water so that the ratio by weight to this powder may be 1:1, and adding binder such as polyvinyl alcohol for increasing the strength of the coat film by about 0.1 wt. %.

Consequently, the calcined material on which the slurry for forming a high-resistance layer is applied is baked in air at 1200° C. to obtain sinter, and both ends of the sinter is polished to form an Al sprayed electrode, thereby obtaining a zinc oxide varistor having a lateral high-resistance layer.

In this conventional method, as the slurry for forming a high-resistance layer, ZnFe₂ O₄ and Zn₇ Sb₂ O₁₂ preliminarily synthesized at high temperature are used, and when a lateral high-resistance layer is formed by using them, the reactivity of the calcined material with ZnFe₂ O₄ and Zn₇ Sb₂ O₁₂ is not sufficient, and the contact between the sinter and the lateral high-resistance layer is poor, and the lateral high-resistance layer is likely to be peeled off during discharge current withstand test, and hence the discharge current withstand capacity characteristic is low.

DISCLOSURE OF THE INVENTION

It is hence an object of the invention to present a zinc oxide varistor excellent in reliability including discharge current withstand capacity characteristic.

To achieve the object, the invention forms a lateral high-resistance additive for zinc oxide varistor by using a metal oxide comprising 1-40 molar % (in terms of Fe₂ O₃) iron, 0-20 molar % (in terms of Bi₂ O₃) bismuth, and the balance consisting of SiO₂.

This lateral high-resistance additive is applied and baked on a lateral face of a molded or calcined material containing zinc oxide as principal ingredient and at least antimony as auxiliary ingredient to form a high-resistance layer on the lateral face of the zinc oxide varistor, and therefore the iron, bismuth and silicon in the lateral high-resistance additive react very well with the ingredients in the molded or calcined material to produce a high-resistance layer containing Zn₂ SiO₄ as principal ingredient and at least Zn₇ Sb₂ O₁₂ dissolving Fe as auxiliary ingredient. This high-resistance layer is homogeneous and excellent in contact with the sinter, and hence discharge current withstand capacity characteristic and other properties can be enhanced substantially.

Moreover, since this lateral high-resistance additive is also extremely excellent in reactivity with the molded material, it can be directly applied on the molded material, and the conventional calcining process of molded material can be omitted, and the loss in time and energy can be saved, so that the productivity may be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a zinc oxide varistor in an embodiment of the invention, and

FIG. 2 is an X-ray diffraction data diagram of zinc oxide varistor in an embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring now to the drawings, a zinc oxide varistor and its manufacturing method, and a lateral high-resistance additive of the zinc oxide varistor according to an embodiment of the invention are described below.

(Embodiment 1)

Supposing the total amount of powdery raw material to be 100 molar % for the principal ingredient of ZnO powder, weighing auxiliary ingredients by 0.5 molar % of Bi₂ O₃, 0.5 molar % of Co₂ O₃, 0.5 molar % of MnO₂, 1.0 molar % of Sb₂ O₃, 0.5 molar % of Cr₂ O₃, 0.5 molar % of NiO, 0.5 molar % of SiO₂, 5×10⁻³ molar % of Al₂ O₃, and 2×10⁻² molar % of B₂ O₃, further adding purified water, binder and dispersant, they were mixed sufficiently in a ball mill and slurry was obtained. From the viewpoint of dispersion, B₂ O₃ is preferred to be added in a form of glass such as bismuth borosilicate or lead borosilicate. As the binder, polyvinyl alcohol (PVA) is preferably added by 1 wt. % of the solid matter from the viewpoint of molding performance, or the dispersant should be added by about 0.5 wt. % of the solid matter from the viewpoint of slurry dispersion.

This slurry was dried and granulated by using a spray dryer, and granulated powder was obtained. The granulated powder was compressed and molded at a pressure of 500 kg/cm² in a size of 40 mm in diameter and 40 mm in thickness by a hydraulic press, and a molded material was obtained.

Next, a lateral high-resistance additive was prepared in the following method. As raw materials of the lateral high-resistance additive, SiO₂, Bi₂ O₃, and Fe₂ O₃ were weighed as specified, and lateral additives of various compositions were prepared. As an organic binder, 5 wt. % of PVA aqueous solution was used. The solid matter ratio of metal oxide was 30 wt. %, and mixing sufficiently in a ball mill together with the binder, a slurry lateral high-resistance additive was prepared. At this time, to enhance the dispersion of the lateral high-resistance additive slurry, it is preferred to add a surface active agent by 0.1 to 0.5 wt. %.

On the lateral portion of the prepared molded material, the lateral high-resistance additive was applied by spray coating method. At this time, while rotating, the molded material was moved up and down, and the lateral high-resistance additive was sprayed so as to be applied uniformly on the molded material. The coating amount of the lateral high-resistance additive on the molded material was 15 mg/cm². Herein, the coating amount of the lateral high-resistance additive is preferably 5 to 100 mg/cm², and more preferably 7.5 to 50 mg/cm². The reason is, if the coating amount of the lateral high-resistance additive is less than 5 mg/cm², the thickness of the lateral high-resistance additive of the zinc oxide varistor element is too thin, and high current short duration characteristic is low, or if exceeding 100 mg/cm², the reactivity between the lateral high-resistance additive and element is worsened, and an unreacted portion is left over to lower also the high current short duration characteristic. To evaluate the performance of the lateral high-resistance additive itself of the invention, the molded material was calcined for 5 hours at 900° C. to prepare a calcined material, and the lateral high-resistance additive was applied in the same process.

The molded material and calcined material coated with lateral high-resistance additive were put in a baking container, and baked for 5 hours at 1100° C. to bake the molded material and calcined material, and sinter was obtained by reaction between the lateral high-resistance additive and the lateral portion of the molded material and calcined material. The sinter was heated for 1 hour at 550° C. Herein, the heating condition of the sinter is preferably 500 to 600° C. The reason is, if lower than 500° C., there is no effect of heat treatment and the high temperature electric charge life characteristic is impaired, or if exceeding 600° C., the voltage nonlinearity is extremely lowered and the high temperature electric charge life characteristic is also impaired. When heating the sinter, preferably, by printing crystalline glass paste of high resistance mainly composed of PbO to the lateral face of the sinter, if there is a defect in the lateral high-resistance layer, it is compensated for, and thickness fluctuation is eliminated, and the high temperature electric charge life, high current short duration characteristic and other reliability are improved. Later, polishing the both ends of the sinter, an aluminum sprayed electrode was formed, and a zinc oxide varistor was obtained. FIG. 1 shows a sectional view of a zinc oxide varistor according to an embodiment of the invention. In FIG. 1, reference numeral 1 is a sinter mainly composed of zinc oxide, 2 is a lateral high-resistance layer formed on a lateral face of the sinter 1, and 3 is an electrode formed at both ends of the sinter 1.

As comparative examples, a molded material obtained in the same process as in the invention, and an element pre-shrunk by calcining the molded material for 5 hours at 900° C. were prepared. The element was coated with a lateral high-resistance additive composed of ZnFe₂ O₄ and Zn₇ Sb₂ O₁₂. Herein, ZnFe₂ O₄ and Zn₇ Sb₂ O₁₂ were preliminarily synthesized at 1100° C. according to the publication cited above. To prepare the lateral high-resistance additive, ZnFe₂ O₄ and Zn₇ Sb₂ O₁₂ were weighed so that the ratio of Fe and Sb might be 2:1, and purified water was added to this powder at 1:1, and to increase the strength of the coat film, PVA was added by 0.1 wt. % as binder, and the obtained lateral high-resistance additive was applied. The coating amount of the lateral high-resistance additive was 15 mg/cm² same as in the invention. By baking, forming electrode and heating-in the same process condition as in the invention, zinc oxide varistors of comparative examples were obtained.

Table 1 shows the composition of lateral high-resistance additive, visual state of appearance, voltage ratio characteristic (V₁ mA /V₁₀ μA), limiting voltage ratio characteristic, discharge current withstand capacity characteristic, and high temperature electric charge life characteristic of examples of the invention and examples of the prior art.

Herein, V₁ mA and V₁₀ μA were measured by using a constant DC current power source. The limiting voltage ratio characteristic was measured in the impulse current condition of 2.5 kA of standard waveform of 8/20 μs. To evaluate the discharge current withstand capacity characteristic, impulse of 50 KA of standard waveform of 4/10 μs was applied twice at an interval of 5 minutes, and abnormality in appearance was observed visually or by using a microscope as required. Later, the current was increased at 10 KA steps, and the breakdown limit was checked. To determine the high temperature electric charge life characteristic, at ambient temperature of 130° C. and charge rate of 95% AVR, the time until the resistance portion leakage current reached a double figure of the initial value was measured.

As clear from Table 1, according to the embodiment, the zinc oxide varistor can be extremely enhanced in the high current short duration characteristic by using SiO₂ mainly in the composition of the lateral high-resistance additive, and adding Fe₂ O₃ by 1 to 40 molar % of the total amount. Further, by controlling the concentration range of Fe₂ O₃ to 3 to 30 molar %, a further stable and excellent high current short duration characteristic can be obtained.

                                      TABLE 1                                      __________________________________________________________________________     Composition of lateral high-                                                                             Electric characteristic                                                                      High current short duration                                                                  High temperature         Sample                                                                             resistance additive (molar %)                                                                              Limiting                                                                             characteristic  electric charge                                                                life                     No. Fe.sub.2 O.sub.3                                                                    Bi.sub.2 O.sub.2                                                                    SiO.sub.2                                                                           Appearance                                                                            V.sub.1 mA /V.sub.10 μA                                                           voltage ratio                                                                        50KA                                                                               60KA                                                                               70KA                                                                               80KA                                                                               characteristic           __________________________________________________________________________                                                           (Hr)                     *101                                                                               0.1  0    99.9 Uneven 1.38  1.60  ∘                                                                    x             400                            reaction                                                                   102       1        0        99    Favorable     1.25         1.63                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                             750                                                                103       3                                                                   0        97                                                                    Favorable     1.26                                                                    1.62                                                                    ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                           700                                                                  104      10                                                                   0        90                                                                    Favorable     1.25                                                                    1.61                                                                    ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘  x                                                                      700                 *105      30        0        70    Favorable     1.26         1.64                                                                  ∘                                                                 ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘  x                                                                   850                    106      40        0        60    Favorable     1.29         1.62                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                       800                 *107      50        0        50    Favorable     1.32         1.58                                                                  ∘ x                                                                       600                                                                     108       3                                                                   1        96                                                                    Favorable     1.21                                                                    1.59                                                                    ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                    900                    109      40        1        59    Favorable     1.25         1.60                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                    >1000                  110      10       15        75    Favorable     1.20         1.62                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                             >1000                                                              111       3                                                                   20        77                                                                   Favorable     1.21                                                                    1.61                                                                    ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                    >1000                  112      30       20        50    Favorable     1.25         1.62                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                              >1000        *113      30       30        40    Favorable     1.24         1.64                                                                  ∘ x                                                                        >1000           *114                                                                               ZnFe.sub.2 O.sub.4 :90                                                                (Molded material                                                                       Uneven 1.36  1.65  ∘                                                                    x             600                                 Zn.sub.7 Sb.sub.2 O.sub.12 :10 application)     reaction                                                                   *115   ZnFe.sub.2                                                             O.sub.4 :50 (Molded                                                            material Uneven                                                                1.33        1.64                                                                  ∘ x                                                                          700                                                                      Zn.sub.7                                                                Sb.sub.2 O.sub.12                                                              :50 application)                                                                reaction                116 Application of composition                                                                    Favorable                                                                             1.23  1.60  ∘                                                                    ∘                                                                    ∘                                                                    ∘                                                                    ∘                                                                    ∘                                                                    x   850                              No. 104 on calcined material                                             117   Application of composition    Favorable    1.19        1.62                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                         800                                                                          No. 110 on                                                              calcined material                                                               *118   Application                                                            of composition                                                                 Favorable    1.32                                                                   1.64                                                                      ∘                                                                  ∘  x                                                                 650                            No. 114 on calcined material                                           __________________________________________________________________________      *Comparative example, different from the invention.                            ∘  No abnormality                                                  x  Broken                                                                

This is because Fe reacts with Zn and Sb at low temperature to form stable substances. Moreover, by adding Bi₂ O₃ in a range of 20 molar % or less, the high temperature electric charge life characteristic can be enhanced. This is because Bi prevents scattering from inside to outside of the sinter. Although 1 molar % or more of Bi₂ O₃ improves the electric charge life characteristic of the lateral high resistance additive and enhances the reactivity, if exceeding 20 molar %, the high current short duration characteristic is lowered. In the prior art, since ZnFe₂ O₄ and Zn₇ Sb₂ O₁₂ are used as the lateral high-resistance additive, the reactivity with the sinter is poor, and the lateral high-resistance additive cannot be applied to the molded material, whereas, in the embodiment, using Fe₂ O₃ and Bi₂ O₃, in addition to the principal ingredient of SiO₂, the reaction activity is high, and the lateral high-resistance additive can be applied to the molded material, and the conventionally required calcining process can be omitted.

In thus obtained zinc oxide varistor, the crystal structure of the lateral high-resistance layer was analyzed by X-ray diffraction. As a representative example, the X-ray diffraction result of the lateral high-resistance layer of the element of sample number 10 is shown in FIG. 2. The principal ingredient of the lateral high-resistance layer is Zn₂ SiO₄, and the auxiliary ingredient is not a mixed crystal of Zn₇ Sb₂ O₁₂ and ZnFe₂ O₄, but is an intermediate state, that is, a single crystal layer in a solid solution state of Fe in Zn₇ Sb₂ O₁₂. As a result of analysis by X-ray micro-analyzer, Sb and Fe were found to be present on a same point. Moreover, the structure of the lateral high-resistance layer was confirmed to be close to a two-layer structure, with Zn₂ SiO₄ existing in the surface, and Zn₇ Sb₂ O₁₂ dissolving Fe existing at the sinter side. It seems because the structure is stable, the adhesion of Zn₇ Sb₂ O₁₂ dissolving Fe and sinter is strong and the dielectric strength of ZnFe₂ O₄ is high, to explain why zinc oxide varistor of the invention is excellent in the high current short duration characteristic. Herein, Zn and Sb detected from the lateral high-resistance layer are derived from ZnO and Sb₂ O₃ in the composition of the molded material, diffusing into the element surface by sintering reaction.

Moreover, in the composition region of the lateral high-resistance layer excellent in high current short duration characteristic, the amount of Fe contained in Zn₇ Sb₂ O₁₂ is 10 to 50 molar % of the amount of Sb. Above all, in the composition regions particularly excellent in the short wave tail tolerance characteristic (sample numbers 4, 6, 8, 10), it is 20 to 40 molar %. The amount of Zn₂ SiO₄ in the lateral high-resistance layer was found to be 98 to 70 molar % by X-ray micro-analyzer and image analysis.

Samples 116 top 118 in Table 1 show data of using the lateral high-resistance additive of the invention in the calcined material. As far as SiO₂, Bi₂ O₃ and Fe₂ O₃ are within the scope of the claims of the invention, it is known that zinc oxide varistors excellent in high current short duration characteristic and high temperature electric charge life characteristic can be obtained same as when applied on the molded material. Therefore, since the lateral high-resistance additive of the invention is excellent in reactivity with the element, both molded material and calcined material can be used. Herein, when calcining, from the viewpoint of working efficiency when applying the lateral high-resistance additive, the shrinkage rate of the calcined material is preferred to be 10% or less, more preferably 5% or less. The reason is, if the shrinkage rate of the molded material is 10% or less, multiple oven pores are present in the calcined material, and when the lateral high-resistance additive is applied, moisture is promptly absorbed in the calcined material. When the shrinkage rate of the calcined material is 5% or less, the moisture is absorbed more efficiently, and the working efficiency is enhanced. On the other hand, when the shrinkage rate exceeds 10%, the sintering reaction is encouraged, the oven pores decrease, and moisture in the lateral high-resistance additive is less absorbed in the calcined material, and the working efficiency is impaired.

(Embodiment 2)

A second embodiment of the invention is described below. Granulated powder of zinc oxide varistor prepared in the same process as in embodiment 1 was molded into a size of 40 mm in diameter and 40 mm in thickness by a hydraulic press. As lateral high-resistance additive, SiO₂, Bi₂ O₃, Fe₂ O₃, and Mn₃ O₄ were weighed as specified, and various lateral high-resistance additives were prepared, and applied on the molded material. At this time, the solid matter ratio of the organic binder and metal oxide was same as in embodiment 1. The application method was spray coating, and the coating amount was 15 mg/cm². The conditions after the baking process of the molded material were same as in embodiment 1, and samples of zinc oxide varistors were prepared.

Table 2 shows the composition of lateral high-resistance additive, voltage ratio characteristic, limiting voltage ratio characteristic, discharge current withstand capacity characteristic, and high temperature electric charge life characteristic according to the second embodiment of the invention.

As clear from Table 2, in the zinc oxide varistor according to the embodiment, using SiO₂ as the principal ingredient of the lateral high-resistance additive, when Fe₂ O₃ is added by 1 to 40 molar % of the whole amount, Bi₂ O₂ by 20 molar % or less, and Mn₃ O₄ by 0.1 to 10 molar %, a zinc oxide varistor excellent in voltage ratio characteristic and high temperature electric charge life characteristic as compared with embodiment 1 is obtained. In particular, when the addition of Mn₃ O₄ is in a range of 0.5 to 5 molar %, the characteristics are particularly excellent including discharge current withstand capacity characteristic. The reason is, it seems, Mn₃ O₄ is dissolved, together with Fe, in Zn₇ Sb₂ O₁₂ in the lateral high-resistance layer to enhance the stability of Zn₇ Sb₂ O₁₂.

(Embodiment 3)

A third embodiment of the invention is described below. Granulated powder of zinc oxide varistor prepared in the same

                                      TABLE 2                                      __________________________________________________________________________     Composition of lateral high-                                                                             Electric characteristic                                                                    High current short duration                                                                    High temperature         Sample                                                                             resistance additive (molar %)                                                                              Limiting                                                                             characteristic  electric charge                                                                life                     No. Fe.sub.2 O.sub.3                                                                   Bi.sub.2 O.sub.3                                                                   SiO.sub.2                                                                          Mn.sub.2 O.sub.4                                                                   Appearance                                                                           V.sub.1 mA /V.sub.10 μA                                                           voltage ratio                                                                        50KA                                                                               60KA                                                                               70KA                                                                               80KA                                                                               characteristic           __________________________________________________________________________                                                           (Hr)                     *201                                                                               0.1 0   98.9                                                                               1   Uneven                                                                               1.28  1.60  x               500                             reaction                                                                  202       1          0       98          1    Favorable      1.22                                                                    1.63                                                                         ∘                                                                  ∘                                                                  ∘  x                                                                            950                                                                203       3                                                                     0       96                                                                     1    Favorable                                                                 1.25         1.62                                                                   ∘                                                              ∘                                                               ∘                                                                  ∘ x                                                                          800                                                                   204      10                                                                     0       90                                                                     0    Favorable                                                                 1.25         1.61                                                                   ∘                                                              ∘                                                               ∘                                                                  ∘                                                                  ∘                                                                  ∘    x                                                                  700                   205      10          0       89.95       0.05 Favorable      1.26                                                                    1.64                                                                         ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘  x                                                                  750                     206      10          0       89.9        0.1  Favorable      1.24                                                                    1.64                                                                         ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                      800                  207      10          0       89.5        0.5  Favorable      1.20                                                                    1.59        0                                                                ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                   >1000                   208      10          0       85          5    Favorable      1.15                                                                    1.58                                                                         ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘  x                                                                >1000                     209      10          0       80        10     Favorable      1.16                                                                    1.60                                                                         ∘                                                                  ∘                                                                  ∘                                                                  ∘   x                                                                       >1000                                                                  *210  10                                                                      0       75                                                                     15     Favorable                                                                 1.16         1.62                                                                   ∘                                                              x                                                                                   >1000                                                                   211      20                                                                     1       78                                                                     1    Favorable                                                                 1.26         1.64                                                                   ∘                                                             ∘                                                                ∘                                                                  ∘   x                                                                          500                                                                 212      20                                                                     5       74                                                                     1    Favorable                                                                 1.23         1.61                                                                   ∘                                                             ∘                                                                ∘                                                                  ∘                                                                  ∘  x                                                                      >1000                                                                    213      20                                                                    10       65                                                                     5    Favorable                                                                 1.19         1.63                                                                   ∘                                                             ∘                                                                ∘                                                                  ∘                                                                  ∘  x                                                                        >1000                                                                  *214  20                                                                      10       55                                                                    15     Favorable                                                                 1.21         1.64                                                                   ∘                                                              x                                                                                   750                                                                     *215  30                                                                      30       35    5                                                                     Favorable                                                                 1.24         1.63                                                                   ∘                                                              x                                                                                   450               __________________________________________________________________________      *Comparative example,  different from the invention.                           ∘ No abnormality                                                   x Broken                                                                 

process as in embodiment 1 was molded into a size of 40 mm in diameter and 40 mm in thickness by a hydraulic press. As lateral high-resistance additive, SiO₂, Bi₂ O₃, Fe₂ O₃, and Al₂ O₃ were weighed as specified, and various lateral high-resistance additives were prepared. At this time, the solid matter ratio of the organic binder and metal oxide was same as in embodiment 1. The application method was spray coating, and the coating amount was 15 mg/cm². The conditions after the baking process of the molded material were same as in embodiment 1, and samples of zinc oxide varistors were prepared.

Table 3 shows the composition of lateral high-resistance additive, voltage ratio characteristic, limiting voltage ratio characteristic, discharge current withstand capacity characteristic, and high temperature electric charge life characteristic according to the third embodiment of the invention.

As clear from Table 3, in the zinc oxide varistor according to the embodiment, using SiO₂ as the principal ingredient of the lateral high-resistance additive, when Fe₂ O₃ is added by 1 to 40 molar % of the whole amount, Bi₂ O₂ by 20 molar % or less, and Al₂ O₃ by 0.01 to 5 molar %, a zinc oxide varistor excellent in limiting voltage ratio characteristic and discharge tolerance characteristic as compared with embodiment 1 is obtained. In particular, when the addition of Al₂ O₃ is in a range of 0.1 to 2.5 molar %, the characteristics are particularly excellent including the high temperature electric charge life characteristic. The reason is, it

                                      TABLE 3                                      __________________________________________________________________________     Composition of lateral high-                                                                             Electric characteristic                                                                    High current short duration                                                                    High temperature         Sample                                                                             resistance additive (molar %)                                                                              Limiting                                                                             characteristic  electric charge                                                                life                     No. Fe.sub.2 O.sub.3                                                                   Bi.sub.2 O.sub.3                                                                   SiO.sub.2                                                                          Al.sub.2 O.sub.3                                                                   Appearance                                                                           V.sub.1 mA /V.sub.10 μA                                                           voltage ratio                                                                        50KA                                                                               60KA                                                                               70KA                                                                               80KA                                                                               characteristic           __________________________________________________________________________                                                           (Hr)                     *301                                                                               0.1 0   98.9                                                                               1   Uneven                                                                               1.30  1.61  x               400                             reaction                                                                  302       1         0       98         1      Favorable     1.28                                                                   1.55                                                                           ∘                                                                  ∘                                                                  ∘ x                                                                           550                                                                  303       3                                                                    0       96                                                                    1      Favorable                                                                1.29      1.56                                                                    ∘                                                              ∘                                                                  ∘                                                                  ∘ x                                                                         500                                                                    304      10                                                                    0       90                                                                    0      Favorable                                                                1.25      1.61                                                                    ∘                                                              ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                     700                   305      10         0       89.99      0.01   Favorable     1.27                                                                   1.58                                                                           ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                     600                   306      10          0       89.9      0.1    Favorable     1.25                                                                   1.55                                                                           ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                     750                   307      10          0       89.5      0.5    Favorable     1.26                                                                   1.53                                                                           ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                   850                     308      10          0       87.5      2.5    Favorable     1.25                                                                   1.54                                                                           ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                   800                     309      10          0       85         5     Favorable     1.31                                                                   1.56                                                                           ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                         450                                                                    *310  10                                                                      0       82.5                                                                   7.5    Uneven                                                                  1.42      1.58                                                                    ∘ x                                                              50                             reaction                                                                  311      20         1       78         1      Favorable     1.26                                                                   1.57                                                                           ∘                                                                  ∘                                                                  ∘                                                                  ∘  x                                                                        500                                                                    312      20                                                                    5       74                                                                    1      Favorable                                                                1.23      1.56                                                                    ∘                                                              ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                  >1000                    313      20         10      67.5       2.5    Favorable     1.29                                                                   1.55                                                                           ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘  x                                                                 550                      *314    20       10      60       10       Uneven        1.45      1.60                                                                    x                                                                                  50                                                                       reaction                                                                  *315    30                                                                    30      35                                                                     5      Favorable                                                                1.38      1.59                                                                    ∘ x                                                            250                      __________________________________________________________________________      *Comparative example, different from the invention.                            ∘ No abnormality                                                   x Broken                                                                 

seems, Al₂ O₃ is diffused in the lateral face of the sinter through the lateral high-resistance layer to be dissolved in ZnO to lower the specific resistance, thereby enhancing the limiting voltage ratio characteristic and the discharge tolerance characteristic.

(Embodiment 4)

A fourth embodiment of the invention is described below. Granulated powder of zinc oxide varistor prepared in the same process as in embodiment 1 was molded into a size of 40 mm in diameter and 40 mm in thickness by a hydraulic press. As lateral high-resistance additive, SiO₂, Bi₂ O₃, Fe₂ O₃, and B₂ O₃ were weighed as specified, and various lateral high-resistance additives were prepared. At this time, the organic binder was 5 wt. % aqueous acrylic (hereinafter called MMAC). The solid matter ratio of the metal oxide was same as in embodiment 1. The application method was spray coating, and the coating amount was 15 mg/cm². The conditions after the baking process of the molded material were same as in embodiment 1, and samples of zinc oxide varistors were prepared.

Table 4 shows the composition of lateral high-resistance additive, voltage ratio characteristic, limiting voltage ratio characteristic, discharge current withstand capacity characteristic, and high temperature electric charge life characteristic according to the fourth embodiment of the invention.

As clear from Table 4, in the zinc oxide varistor according to the embodiment, using SiO₂ as the principal ingredient of the

                                      TABLE 4                                      __________________________________________________________________________     Composition of lateral high-                                                                             Electric characteristic                                                                    High current short duration                                                                    High temperature         Sample                                                                             resistance additive (molar %)                                                                              Limiting                                                                             characteristic  electric charge                                                                life                     No. Fe.sub.2 O.sub.3                                                                   Bi.sub.2 O.sub.3                                                                   SiO.sub.2                                                                          B.sub.2 O.sub.3                                                                    Appearance                                                                           V.sub.1 mA /V.sub.10 μA                                                           voltage ratio                                                                        50KA                                                                               60KA                                                                               70KA                                                                               80KA                                                                               characteristic           __________________________________________________________________________                                                           (Hr)                     *401                                                                               0.1 0   98.9                                                                               1   Uneven                                                                               1.28  1.63  x               550                             reaction                                                                  402      1          0       98        1    Favorable     1.23                                                                      1.64       .smallcir                                                           cle. ∘                                                             ∘ x                                                                         >1000                                                                  403      3                                                                     0       96                                                                    1    Favorable                                                                 1.24          1.62                                                                  ∘                                                             ∘                                                                  ∘                                                                  ∘ x                                                                            850                                                                 404     10                                                                     0       90                                                                    0    Favorable                                                                 1.25          1.60                                                                  ∘                                                             ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                        650                                                                     405     10                                                                     0       89.99                                                                 0.01 Favorable                                                                 1.25          1.64                                                                  ∘                                                             ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                        800                                                                     406     10                                                                     0       89.95                                                                 0.05  Favorable                                                                1.24          1.62                                                                  ∘                                                             ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                    >1000                  407     10          0       89.5      0.5  Favorable     1.22                                                                      1.62       .smallcir                                                           cle. ∘                                                             ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                  >1000                    408     10          0       87.5      2.5  Favorable     1.20                                                                      1.60       .smallcir                                                           cle. ∘                                                             ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘ x                                                                  >1000                    409     10          0       85        5    Favorable     1.18                                                                      1.64       .smallcir                                                           cle. ∘                                                             ∘ x                                                                          >1000                                                                 *410  10         0                                                                  82.5      7.5                                                             Uneven        1.23                                                                     1.63                                                                  x                                                                                >1000                         reaction                                                                  411     20          1       78       1    Favorable      1.24                                                                      1.62       .smallcir                                                           cle. ∘                                                             ∘                                                                  ∘ x                                                                       850                 412     20          5       74       1    Favorable      1.24                                                                      1.61       .smallcir                                                           cle. ∘                                                             ∘                                                                  ∘ x                                                                       >1000                                                                    413     20                                                                    10       65       5                                                               Favorable                                                                   1.20          1.66                                                                  ∘                                                             ∘                                                                  ∘                                                                  ∘ x                                                                       >1000                                                                    *414      20    10                                                                  62.5     7.5                                                             Uneven         1.26                                                                     1.70                                                                  x                                                                                550                           reaction                                                                  *415      30    30       39       1    Favorable      1.24                                                                         1.64       .smallcir                                                           cle. x                                                                              650                 __________________________________________________________________________      *Comparative example, different from the invention.                            ∘ No abnormality                                                   x Broken                                                                 

lateral high-resistance additive, when Fe₂ O₃ is added by 1 to 40 molar % of the whole amount, Bi₂ O₂ by 20 molar % or less, and B₂ O₃ by 0.1 to 5 molar %, a zinc oxide varistor excellent in voltage ratio characteristic and high temperature electric discharge life characteristic as compared with embodiment 1 is obtained. In particular, when the addition of B₂ O₃ is in a range of 0.5 to 2.5 molar %, the characteristics are particularly excellent including the discharge current withstand capacity characteristic. The reason of enhancement of high temperature electric charge life characteristic by addition of B₂ O₃ is, it seems, B₂ O₃ is diffused in the lateral face of the sinter through the lateral high-resistance layer to increase the stability of the grain boundary area.

Incidentally, when B₂ O₃ is added in a form of glass such as bismuth borosilicate and lead borosilicate, it is confirmed that the high temperature electric charge life characteristic is enhanced. The reason of adding in glass form is, when using PVA as binder, because B₂ O₃ and binder solution react to increase extremely the viscosity of the lateral high resistance additive, and it is intended to prevent this phenomenon.

(Embodiment 5)

A fifth embodiment of the invention is described below. Granulated powder of zinc oxide varistor prepared in the same process as in embodiment 1 was molded into a size of 40 mm in diameter and 40 mm in thickness by a hydraulic press. The composition of the lateral high-resistance additive is the lateral high-resistance additive used in sample number 4 in embodiment 1, that is, a composition of 90 molar % of SiO₂ and 10 molar % of Fe₂ O₃, and a lateral high-resistance additive in a slurry form was prepared. The lateral high-resistance additive was prepared at a solid matter ratio of 25% by using 5 wt. % methyl cellulose (hereinafter called MC) as the binder, and it was applied on the lateral face of the molded material by a curvature screen printing method. Consequently, the molded material coated with the lateral high-resistance additive was put in a baking container, and baked for 5 hours at 900 to 1300° C. to sinter the element, while the lateral high-resistance additive and the lateral face of the molded material were reacted to obtain a sinter. Then, by the same process as in embodiment 1, the zinc oxide varistor was obtained.

To obtain comparative examples, on the molded material obtained in the same process as in embodiment 1, and the element obtained by pre-shrinking by calcining for 5 hours at 900° C., the lateral high-resistance additive composed of ZnFe₂ O₄ and Zn₇ Sb₂ O₁₂ was applied, and baked, and samples were prepared.

Table 5 shows the evaluation results of appearance of the sinter, V₁ mA/mm (varistor voltage per unit thickness), high current short duration characteristic, and low current long duration characteristic of the zinc oxide varistor obtained in this manner.

Herein, to evaluate the low current long duration characteristic, a rectangular wave current of 2 ms was applied 20 times at intervals of 2 minutes and the appearance was investigated. The

                                      TABLE 5                                      __________________________________________________________________________         Lateral high-                                                                        Calcining                                                                            Bakin            High current short duration                                                                    Low current long                                                               duration                        Sample   resistance    of molded tempera-  Appearance   characteristic                                                        haracteristic                 No. additive                                                                             material                                                                             ture (° C.)                                                                  of sinter                                                                            V.sub.1 mA/mm                                                                        40KA                                                                               50KA                                                                               60KA                                                                               70KA                                                                               50A 100A                                                                              150A                                                                               200A               __________________________________________________________________________     *501                                                                               No. 104                                                                              No    900  Partly                                                                               800   ∘                                                                    x             ∘                                                                      x                               unreacted                                                                  502    No. 104      No        950   Favorable    500   ∘                                                                     ∘                                                                   x                                                                             ∘                                                                   ∘                                                                  x                   503    No. 104      No       1000   Favorable    350   ∘                                                                     ∘                                                                   ∘                                                                   .smallcircle                                                                 .  x                                                                           ∘                                                                  ∘                                                                  ∘                                                                  x                    504    No. 104      No       1200   Favorable    200   ∘                                                                     ∘                                                                   ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘       505    No. 104      No       1300   Favorable    170   ∘                                                                     ∘                                                                   ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  x .smallcircl                                                                 e. .smallcircl                                                                 e. .smallcircl                                                                 e. .smallcircl                                                                 e.                   *506   No. 104   No       1350   Partly       160   ∘                                                                        ∘                                                                   ∘                                                                  x                                                                             ∘                                                                  ∘                                                                  ∘                                                                   ∘           scattered                                                                  507    No. 104      Yes       950   Favorable    450   ∘                                                                     ∘                                                                   x                                                                             ∘                                                                  ∘                                                                         x                                                                        508    No.                                                                    104      Yes                                                                       1200                                                                       Favorable                                                                      190                                                                            ∘                                                                    .smallcircle                                                                 .  .smallcircl                                                                 e. .smallcircl                                                                 e. .smallcircl                                                                 e. .smallcircl                                                                 e. .smallcircl                                                                 e. x .smallcir                                                                 cle. .smallcir                                                                 cle. .smallcir                                                                 cle. .smallcir                                                                 cle.                 509    No. 104      Yes      1300   Favorable    165   ∘                                                                     ∘                                                                   ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  x  .smallcirc                                                                 le. .smallcirc                                                                 le. .smallcirc                                                                 le. .smallcirc                                                                 le.                  *510     No. 115 No        900   Partly       800   x          .smallcir                                                                 cle.      x                                                                         unreacted       *511     No. 115      No        950   Favorable    500   ∘                                                                    x                                                                             ∘                                                                  x                    *512     No. 115      No       1200   Favorable    200   ∘                                                                    ∘                                                                    .smallcircl                                                                 e.  .smallcirc                                                                 le.  x                                                                         ∘                                                                    .smallcircle                                                                 .  .smallcircl                                                                 e. x                 *513     No. 115      Yes       950   Favorable    450   ∘                                                                    x                                                                             ∘                                                                  x                    *514     No. 115      Yes      1200   Favorable    190   ∘                                                                    ∘                                                                   .smallcircle                                                                 . .smallcircle                                                                 . x                                                                            ∘                                                                  ∘                                                                  ∘                                                                  x                  __________________________________________________________________________      *Comparative example, different from the invention.                            ∘ No abnormality                                                   x Broken                                                                 

current was started from 50 A, and increased at 50 A steps until the element was broken.

As known from Table 5, when using the lateral high-resistance additive of SiO₂ and Fe₂ O₃, as compared with the comparative examples, it is recognized that the high current short duration characteristic and low current long duration characteristic are excellent on the whole. Herein, if the baking temperature is 900° C., the reactivity of the lateral high-resistance additive and element is poor, and the high current short duration characteristic is low. At 1350° C., on the other hand, part of the lateral high-resistance additive scatters away, and the high current short duration characteristic is poor, too. When baked at low temperature, zinc oxide particles are not grown sufficiently, and V₁ mA/mm is too high, and it is not practical as an element for electric power. Therefore, the baking temperature is preferably 950 to 1300° C. More preferably, it should be 1000 to 1200° C. in consideration of the low current long duration characteristic.

(Embodiment 6)

A sixth embodiment of the invention is described below. Granulated powder of zinc oxide varistor prepared in the same process as in embodiment 1 was molded into a size of 40 mm in diameter and 40 mm in thickness by a hydraulic press. At this time, the molding pressure was adjusted so that the density of the molded material might be 3.0 to 3.5 g/cm³. As the lateral high-resistance additive, the lateral high-resistance additive used in sample number 4 in embodiment 1 was used, that is, a composition of 90 molar % of SiO₂ and 10 molar % of Fe₂ O₃.

The lateral high-resistance additive was applied on the lateral face of the prepared molded material by transfer coating method. In transfer coating, the lateral high-resistance additive was preliminarily spread wide thinly on a metal plate by printing, and the molded material was applied by rotating. In this method, the lateral high-resistance additive can be applied easily in a very simple equipment. However, as compared with the spray coating, the coating thickness of the lateral high-resistance additive is slightly uneven, and hence the short wave tail tolerance characteristic fluctuates, but the uniformity can be improved by adjusting the rotating speed of the molded material. Moreover, to improve the mass producibility, the lateral high-resistance additive may be applied on the surface of the rotating roller, and the lateral high-resistance additive may be applied while rotating the molding material. Then, in the same process condition as in embodiment 1, from baking to electrode application, the zinc oxide varistor was obtained. As a comparative example, the lateral high-resistance additive was applied on the calcined material calcined at 950° C., and a sample was prepared by baking.

Table 6 shows the voltage ratio characteristic, limiting voltage characteristic, and low current long duration characteristic of the zinc oxide varistors obtained in the above process.

Herein, the voltage ratio characteristic and limiting

                                      TABLE 6                                      __________________________________________________________________________         Density of molded                Low current long duration                   Sample  material   Calcining of  Electric characteristic  characteristic     No. (g/cm.sup.3)                                                                            molded material                                                                        V.sub.1 mA /V.sub.10 μA                                                           Limiting voltage ratio                                                                   150A                                                                              200A                                                                              250A                                                                              300A                             __________________________________________________________________________     *601                                                                               3.1      No      1.20  1.62      x                                           602        3.15                No           1.21              1.61                                                                 ∘                                                          ∘                                                                  ∘  x                   603        3.2                 No           1.21              1.62                                                                 ∘                                                          ∘                                                                  ∘  x                   604        3.35                No           1.23              1.63                                                                 ∘                                                          ∘                                                                  ∘  ∘       605        3.4                 No           1.24              1.63                                                                 ∘                                                          ∘                                                                  ∘  x                   *606    3.5                 No           1.27              1.65                                                                 x                             *607    2.9                 Yes          1.20              1.60                                                                 ∘      x                                                       608        3.0                                                                    Yes          1.20                                                                 1.61                                                                    ∘      .smallcir                                                   cle.       ∘  x        609        3.4                 Yes          1.22              1.60                                                                 ∘                                                          ∘                                                                  ∘  x                   *610    3.5                 Yes          1.23              1.61                                                                 ∘               __________________________________________________________________________                                                   x                                 *Comparative example, different from the invention.                            ∘  No abnormality                                                  x Broken                                                                 

voltage characteristic were measured in the same conditions as in embodiment 1. Besides, to evaluate the low current long duration characteristic, a rectangular wave current of 2 mS was applied 20 times at intervals of 2 minutes and the appearance was investigated. The current was started from 150 A, and increased at 50 A steps until the element was broken.

As known from Table 6, when applying the lateral high-resistance additive on the molded material, the low current long duration characteristic is excellent when the density is 3.15 to 3.4 g/cm³. That is, if smaller than 3.15 g/cm³, in the manufacturing method of the invention, since the lateral high-resistance additive made from an aqueous binder is applied on the molded material, moisture permeates inside from the lateral face of the molded material, and the binder in the molded material is swollen, and micro-cracks are formed on the surface of the molded material. On the other hand, if greater than 3.4 g/cm³, the binder in the molded material is not burned sufficiently, and cracks and other defects are formed inside the sinter. These problems are lessened by calcining the molded material, and the favorable density range of the molded material for low current long duration characteristic is found to be 3.15 to 3.4 g/cm³. This is because, when the molded material is calcined, the strength of the molded material is improved and micro-cracks are not formed on the surface if the lateral high-resistance additive is applied. If the molded material is calcined, however, when the molded material density is over 3.4 g/cm³, the binder is not burned sufficiently, internal defects occur, and the low current long duration characteristic is impaired.

(Embodiment 7)

A seventh embodiment of the invention is described below. Granulated powder of zinc oxide varistor prepared in the same process as in embodiment 1 was molded into a size of 40 mm in diameter and 40 mm in thickness by a hydraulic press. At this time, the molding pressure was adjusted so that the density of the molded material might be 3.3 g/cm³ As the lateral high-resistance additive, the lateral high-resistance additive used in sample number 11 in embodiment 1 was used, that is, a composition of 77 molar % of SiO₂, 20 molar % of Bi₂ O₃, and 3 molar % of Fe₂ O₃. According to the blending composition, SiO₂, Bi₂ O₃, and Fe₂ O₃ were weighed as specified, and an oxide for lateral high-resistance additive was prepared. As an organic binder, water-soluble PVA, MC, hydroxypropyl cellulose (hereinafter HPC), and MMAC were weighed as specified, and dissolved in purified water. The oxide of the lateral high-resistance additive and the organic binder aqueous solution were weighed, and mixed sufficiently in a ball mill, and a slurry composition of lateral high-resistance additive was obtained. The viscosity of the slurry was adjusted by adding purified water. On the lateral face of the molded material, this lateral high-resistance additive was applied by dip method. In the dip method, the flat portion of the molded material is held by a jig, and is passed through the lateral high-resistance additive. The molded material coated with thus prepared lateral high-resistance additive was treated in the same process as in embodiment 1, and the zinc oxide varistor was obtained.

Table 7 shows the types of lateral high-resistance additive, time to dry to the touch, appearance of sinter, high current short duration characteristic, and low current long duration characteristic.

As known from Table 7, the binder to be used in the lateral high-resistance additive may be any one of PVA, MC, HPC, and MMAC, but the preferred concentration of binder aqueous solution is found to be 1 to 15 wt. %. That is, if the concentration of the binder aqueous solution is low, the coat film strength of the lateral high-resistance additive is low, a sufficient coating amount is not obtained, and the high current short duration characteristic is lowered. If too high, on the other hand, the slurry flow is poor, and it takes a long time to dry and micro-cracks are formed on the surface of the molded material, and hence the high current short duration characteristic and low current long duration characteristic are impaired. The amount of addition of metal oxide for lateral high-resistance additive is preferred to be 15 to 60 wt. % as the solid matter ratio. If the solid matter ratio is low, it takes time to dry and the low current long duration characteristic is impaired, or if the solid matter ratio is too high, the coat film cannot be applied uniformly and the high current short duration characteristic is impaired. Incidentally, the viscosity of the lateral high-resistance

                                      TABLE 7                                      __________________________________________________________________________             Binder                                                                              Solid matter                                                                         Time to dry   High current short duration                                                                    Low current long                                                               duration                        Sample     addition  ratio        to the touch Appearance   characterist                                                      ic characteristic             No. Binder                                                                             (%)  (%)   (sec)  of sinter                                                                             50KA                                                                                60KA                                                                              70KA                                                                               80KA                                                                               150A                                                                               200A                                                                              250A                                                                               300A               __________________________________________________________________________     *701                                                                               PVA 0.1  30    30     Favorable                                                                             ∘                                                                    ∘                                                                    x           ∘                                                                     x                            702  PVA       1       30           25      Favorable  ∘                                                                    ∘                                                                  ∘                                                                  ∘                                                                  x    .smallcirc                                                                le. .smallcircl                                                                e. .smallcircle                                                                . x                   703  PVA       2.5     10           30      Favorable  ∘                                                                    ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  x   .smallcircl                                                                e. .smallcircle                                                                . x                   704  PVA       2.5     15           25      Favorable  ∘                                                                    ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  x  .smallcircle                                                                . ∘                                                                 ∘                                                                 x                     705  PVA       2.5      50           15      Favorable  ∘                                                                   ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  x ∘                                                                 ∘                                                                 ∘                                                                  x                     706  PVA       2.5     60           15      Favorable  ∘                                                                    ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  x   .smallcircl                                                                e. .smallcircle                                                                . ∘                                                                 x                    707  PVA     10        30           25      Favorable  ∘                                                                    ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  x   .smallcircl                                                                e. .smallcircle                                                                . ∘                                                                 x                    708  PVA     15        30           30      Favorable  ∘                                                                    ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  x   .smallcircl                                                                e. .smallcircle                                                                . x                   *709  PVA     30    65           35      Uneven     ∘                                                                       ∘                                                                  x                                                                              ∘                                                                  ∘                                                                  x                          reaction                                                                  710  MC        5       30           25      Favorable  ∘                                                                    ∘                                                                   ∘                                                                 ∘                                                                  ∘                                                                  ∘                                                                   x  .smallcircl                                                                e. .smallcircle                                                                . ∘                                                                 x                    711  MC      10        20           30      Favorable  ∘                                                                    ∘                                                                   ∘                                                                 ∘                                                                  ∘                                                                  x   .smallcircl                                                                e. .smallcircle                                                                . ∘                                                                     x                                                                          712  HPC                                                                      5        30                                                                           25                                                                      Favorable                                                                      ∘                                                                  ∘                                                                   ∘                                                                 ∘                                                                  ∘                                                                  ∘                                                                  x   .smallcircl                                                                e. .smallcircle                                                                .    x                                                                          713  HPC                                                                      10        20                                                                           30                                                                      Favorable                                                                     ∘                                                                  ∘                                                                   ∘                                                                 ∘                                                                  ∘                                                                  ∘                                                                  ∘                                                                  x ∘                                                                 ∘                                                                 ∘                                                                  x                     714  MMAC    5         30           20      Favorable  ∘                                                                    ∘                                                                   ∘                                                                 ∘                                                                  ∘                                                                  ∘                                                                  x  .smallcircle                                                                . ∘                                                                 ∘                                                                     x                 715  MMAC    10        20           25      Favorable  ∘                                                                    ∘                                                                   ∘                                                                 ∘                                                                  ∘                                                                  ∘                                                                  x  .smallcircle                                                                . ∘                                                                 ∘                                                                     x               __________________________________________________________________________      *Comparative example, different from the invention.                            ∘ No abnormality                                                   x  Broken                                                                

additive should be preferably changed depending on the method of application, lower in the spray coating method and higher in the screen printing method. Approximately, a practical viscosity range is 500 to 1000 cps.

INDUSTRIAL APPLICABILITY

According to the invention, as described herein, when the lateral high-resistance additive is applied and baked on the lateral face of a molded material or calcined material, and a high-resistance layer is formed on the lateral face of a zinc oxide varistor, iron, bismuth and silicon in the lateral high-resistance additive react very well with the ingredients in the molded material or calcined material, thereby forming a high-resistance layer comprising Zn₂ SiO₄ as principal ingredient, and at least Zn₇ Sb₂ O₁₂ dissolving Fe as auxiliary ingredient. This high resistance-layer is homogeneous, excellent in adhesion with the sinter, and high in dielectric strength, so that discharge current withstand capacity characteristic and high current short duration characteristic may be substantially enhanced. Moreover, by adding oxides of Mn, Al, B and others to the lateral high-resistance additive, the high temperature electric charge life characteristic and other properties can be enhanced. In addition, since the lateral high-resistance additive is excellent in reactivity with the molded material, it can be directly applied on the molded material, and hence the loss in time and energy can besaved, and the productivity can be enhanced. 

We claim:
 1. A lateral high-resistance additive for zinc oxide varistor having a metal oxide comprising 1-40 molar % (in terms of Fe₂ O₃) iron, 0-20 molar % (in terms of Bi₂ O₃) bismuth, and the balance consisting of SiO₂.
 2. A lateral high-resistance additive for zinc oxide varistor of claim 1, wherein the metal oxide further comprises 0.1 to 10 molar % (in terms of Mn₃ O₄) manganese.
 3. A lateral high-resistance additive for zinc oxide varistor of claim 1, wherein the metal oxide further comprises 0.01 to 2 molar % (in terms of Al₂ O₃) aluminum.
 4. A lateral high-resistance additive for zinc oxide varistor of claim 1, wherein the metal oxide further comprises 0.05 to 5 molar % (in terms of B₂ O₃) boron.
 5. A lateral high-resistance additive for zinc oxide varistor of claim 4, wherein boron is added in a form of glass frit.
 6. A manufacturing method of zinc oxide varistor comprising the steps of compacting a powdery raw material of zinc oxide varistor containing zinc oxide as principal ingredient and at least antimony as auxiliary material to obtain a molded material, applying a lateral high-resistance additive composed of an aqueous binder solution and metal oxide on the lateral face of the molded material, baking the molded material to obtain a sinter, and heating the sinter in a temperature range of 500 to 600° C., wherein the metal oxide comprises 1-40 molar % (in terms of Fe₂ O₃) iron, 0-20 molar % (in terms of Bi₂ O₃) bismuth, and the balance consisting of SiO₂.
 7. A manufacturing method of zinc oxide varistor of claim 6, wherein the baking temperature is in a temperature range of 950 to 1300° C.
 8. A manufacturing method of zinc oxide varistor of claim 6, wherein the density of the molded material is in a range of 3.15 to 3.40 g/cm³.
 9. A manufacturing method of zinc oxide varistor of claim 6, wherein the lateral high-resistance additive is applied in any one of dip coating method, spray coating method, transfer coating method, and curvature screen printing method.
 10. A manufacturing method of zinc oxide varistor of claim 6, wherein the metal oxide further includes at least one selected from the group consisting of manganese, aluminum, and boron.
 11. A manufacturing method of zinc oxide varistor comprising the steps of compacting a powdery raw material for zinc oxide varistor to obtain a molded material, calcining the molded material until its shrinkage rate is 10% or less to obtained a calcined material, applying a lateral high-resistance additive composed of an aqueous binder solution and metal oxide on the lateral face of the calcined material, baking the calcined material to obtain a sinter, and heating the sinter in a temperature range of 500 to 600° C., wherein the metal oxide comprises 1-40 molar % (in terms of Fe₂ O₃) iron, 0-20 molar % (in terms of Bi₂ O₃) bismuth, and the balance consisting of SiO₂.
 12. A manufacturing method of zinc oxide varistor of claim 11, wherein the baking temperature is in a temperature range of 950 to 1300° C.
 13. A manufacturing method of zinc oxide varistor of claim 11, wherein the density of the molded material is in a range of 3.15 to 3.40 g/cm³.
 14. A manufacturing method of zinc oxide varistor of claim 11, wherein the lateral high-resistance additive is applied in any one of dip coating method, spray coating method, transfer coating method, and curvature screen printing method.
 15. A manufacturing method of zinc oxide varistor of claim 11, wherein the metal oxide further includes at least one selected from the group consisting of manganese, aluminum, and boron. 